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S.R. Heap (NASA/GSFC), I. Hubeny (NOAO/GSFC), E. B. Jenkins (Princeton), M. S. Sahu (NOAO/GSFC), A. Smette (NOAO/GSFC), T. M. Tripp (Princeton), G. M. Williger (NOAO/GSFC), HST/STIS IDT
We present a new far-UV spectrum of the z=3.29 QSO, Q0302-003, obtained by STIS. This spectrum traces redshifted HeII Lyman-\alpha \lambda304 absorption over a redshift range, z=2.8 - 3.28, at a resolution of \Delta\lambda =1 Å. In agreement with previous observations by the FOC (Jakobsen et al. 1994) and the GHRS (Hogan et al. 1997), the STIS spectrum clearly shows the ``Gunn-Peterson absorption trough'' formed by redshifted HeII Lyman-\alpha. The STIS spectrum, however, also shows that the HeII opacity is not constant with redshift: there is a hole in the opacity at z=3.05 as well as numerous minor fluctuations in opacity at z<3.05. We will describe the major features of the absorption trough and will compare the observed distribution of opacities with theoretical predictions.